One restriction which is imposed on printing apparatus of this type is that they are normally intended to be portable and are therefore small. This imposes a limitation on the amount of memory which is available within the printing apparatus. These printing apparatus are controlled by a microprocessor chip. It is advantageous if the memory space which is made available on the microprocessor chip is sufficient to store all of the necessary font data and program code for running the printing apparatus. This is discussed in our earlier Application PCT/GB91/02126. This requires that font data which is stored to define the characters to be printed is stored in compressed form. There are many ways in which compression of font data can occur. However, any compression technique which works well for some characters often does not work well for others and can, in some cases, actually increase the amount of memory required to store the character. Therefore, even printing apparatus which do use font compression techniques are not able to reduce the storage capacity which is required to a minimum.
There is, moreover, normally a requirement for reduced size characters, such as half size characters. While it is theoretically quite possible (and some printing apparatus do this) to store font data defining the half size characters separately from font data defining the full size characters, this once again increases the storage requirements of the printing apparatus. An alternative is to derive the half size characters from font data stored for the full sized character. However, these half sized characters vary in their quality of appearance because a common "half size" algorithm is used to produce all of the characters which, while rendering faithful half size versions of some characters distorts others.